1. Field of the Invention
The present invention relates to a nanoemulsion based on an anionic surfactant chosen from phosphoric acid fatty esters and their oxyethylenated derivatives and on at least one oil having a molecular weight of greater than 400, the ratio by weight of the amount of oily phase to the amount of surfactant ranging from 2 to 10.
The invention also relates to a process for the preparation of the nanoemulsion and to its uses, in particular in the cosmetics, dermatological and/or ophthalmological fields. This nanoemulsion is stable on storage and can contain large amounts of oil while retaining good transparency and while having good cosmetic properties.
2. Discussion of the Background
Nanoemulsions are oil-in-water emulsions, the oil globules of which have a very fine particle size, i.e. a number-average size of less than 100 nm. They are generally manufactured by mechanical fragmentation of an oily phase in an aqueous phase in the presence of a surfactant. In the case of nanoemulsions, the very small size of the oily globules is obtained in particular by virtue of at least one pass through a high-pressure homogenizer. The small size of the globules confers on them cosmetically advantageous properties which distinguish them from conventional emulsions: they are transparent and exhibit a novel texture. They can also carry active principles more efficiently.
Transparent microemulsions are known in the art. In contrast to nanoemulsions, microemulsions are not, strictly speaking, emulsions. Rather, microemulsions are transparent solutions of micelles swollen by oil, which oil is generally a very-short-chain oil (e.g. hexane or decane) and which is solubilized by virtue of the joint presence of a significant amount of surfactants and of cosurfactants which form the micelles. The size of the swollen micelles is very small owing to the small amount of oil which they can solubilize. This very small size of the micelles is the cause of their transparency, as with nanoemulsions. However, in contrast to nanoemulsions, microemulsions are spontaneously formed by mixing the constituents, without contributing mechanical energy other than simple magnetic stirring. The major disadvantages of microemulsions are related to their necessarily high proportion of surfactants, leading to intolerance and resulting in a sticky feel during application to the skin. Furthermore, their formulation range is generally very narrow and their temperature stability very limited.
Nanoemulsions, which contain an amphiphilic lipid phase composed of phospholipids, water and oil are known in the art. These emulsions exhibit the disadvantage of being unstable on storage at conventional storage temperatures, namely between 0 and 45.degree. C. They lead to yellow compositions and produce rancid smells which develop after several days of storage.
Nanoemulsions stabilized by a lamellar liquid crystal coating, obtained by the combination of a hydrophilic surfactant and of a lipophilic surfactant, are also known. However, these combinations are difficult to prepare. Furthermore, the nanoemulsions obtained exhibit a waxy and film-forming feel which is not very pleasant for the user.
EP-A-728,460 discloses nanoemulsions based on fluid non-ionic amphiphilic lipids. However, these nanoemulsions disadvantageously exhibit a sticky effect during application to the skin.
The need therefore remains for nanoemulsions which have neither the disadvantages of known nanoemulsions nor the disadvantages of microemulsions.